Similar to "scimitar" turboprop blades.
http://www.cap-ny153.org/E2%20Hawkeye%20Prop%2002.jpg
The outer ends of the blades travel farther per rotation (2pi*(r)) and thus approach Mach 1 at high rpms. Close to Mach 1, supersonic shockwaves start to form. Which 1) interfere with efficiency and 2) produce noise (that "growling sound" heard from many turbofans and props.)
Just as with swept wings, "sweeping" the blades delays the onset of the supersonic shocks.
But with a cowled fan, once the blades are very close to the cowling, the cowling itself reduces/delays shock onset, so the leading edge can re-curve "forward" again, grabbing just a bit more air without producing more shock.
Not new ideas aerodynamically, but the ability to produce the complex blade curvatures with formed composites (instead of machining chunks of titanium) makes it now economical to "tune" the blade shape for each radius, producing the "wavy" leading edge.